1. Field of the Invention
The present invention relates to a liquid crystal display device used for a portable information equipment, a portable phone, and the like, a lighting unit such as a front light or a back light for irradiating a non-self light emitting display element with light, and for a phosphor bead or a phosphor film for converting a wavelength of light from a light source.
2. Description of the Related Art
Liquid crystal display devices providing high-definition color images at low power consumptions have been frequently used as display devices for recent portable phones, mobile computers, and the like. A light source employing a high-luminance white LED has been used as the light source for the lighting units, for illuminating liquid crystal devices used in those liquid crystal display devices.
In particular, a bright reflection type liquid crystal display device or a double-side visible type liquid crystal display device in which image information can be displayed on both front and rear surfaces, has been used in a portable phone. The white LED used for lighting such the display devices has such a construction that a resin having dispersed thereinto a green phosphor or a yellow phosphor is provided onto the light-emitting surface of a blue LED. As a result, green light or yellow light, and the original blue light are mixed, thereby being capable of obtaining white light.
FIG. 15 is an XY chromaticity diagram explaining the color reproduction range of a conventional white LED. A color triangle obtained by connecting the points shown by R, G, and B in the figure is that of a color CRT. Adjusting the luminance of each of those R, G, and B pixels enables all the colors present inside the color triangle to be represented. When the area of the color triangle of the color CRT is set to 100%, the area of a color triangle possessed by a color display device except a CRT is defined as an NTSC ratio, and the ratio is an indicator for color reproducibility. The conventional white LED can reproduce a color only on a straight line connecting chromaticity coordinates 101 of only a blue LED with excitation light and chromaticity coordinates 102 of only yellow light obtained by wavelength conversion, so a color reproduction range is extremely narrow. Of course, a yellow phosphor can reproduce a red color and a green color (although the chroma of each of the colors is low) as well as a yellow color because a green light component and a red light component are also produced by wavelength conversion. An oxide phosphor such as yttrium aluminum garnet (YAG) added with a rare earth element or a chalcogenide compound has been frequently used as a phosphor.
Meanwhile, the intensity of light applied to a phosphor is large when the phosphor is used in the same manner as in the conventional white LED. JP 07-176794 A (Patent Document 1) discloses that a phosphor is applied to and formed on the rear surface of a light guide plate at a predetermined formation density with a view to preventing the light degradation of the phosphor.
Furthermore, JP 10-269822 A (Patent Document 2) discloses that a laminar wavelength converter is arranged between a blue LED and the light incidence plane of a light guide plate so that wavelength conversion can be performed by means of a phosphor with a lower area than that in the method disclosed in the above patent document.
However, a conventional oxide phosphor has a problem in that light conversion efficiency is low. The use of the so-called chalcogenide compound such as a sulfide, a selenide, or a telluride each having high light conversion efficiency for solving the problem involves the emergence of a new problem in that it is difficult to lengthen the lifetime of each of these phosphors owing to the poor moisture resistance of each of the phosphors.
In addition, a conventional lighting unit in which a conventional blue LED, and a green phosphor or a yellow phosphor are combined to provide white light has a problem in that a color reproduction range is narrow owing to a reduction in luminous intensity of a red color.
In addition, a conventional lighting unit in which a phosphor is applied to the rear surface of a light guide plate has a problem in that application conditions must be conformed every time the specifications of the light guide plate are changed because an application concentration must be conformed to a light intensity distribution inside the light guide plate. Furthermore, applying the phosphor to the rear surface of the light guide plate involves the emergence of a problem in that it becomes difficult to design the light guide plate because light propagation property inside the light guide plate changes.
Furthermore, a conventional lighting unit in which a laminar wavelength converter is arranged between a light source and the light incidence plane of a light guide plate has a problem in that color mixing property is bad because a distance between the light source and the wavelength converter is so short that the intensity distribution of light applied to the wavelength converter is large.